In various industries, various efforts to reduce environmental impacts have been made on a global scale. In particular, in the automobile industry, the development of techniques has progressed for the spread of not only a gasoline engine vehicle having superior fuel efficiency but also a so-called eco-car such as a hybrid vehicle or an electric vehicle and for further improvement in the performance of the vehicles. Along with the development of such an eco-car, studies regarding an exhaust gas purification catalyst which purifies exhaust gas exhausted from an engine have been actively made. This exhaust gas purification catalyst includes an oxidation catalyst, a three way catalyst, and a NOx storage reduction catalyst. In the exhaust gas purification catalyst, catalytic activity is exhibited by a noble metal catalyst such as platinum (Pt), palladium (Pd), or rhodium (Rh). In general, this noble metal catalyst is used in a state of being supported on a support formed of a porous oxide such as alumina.
In an exhaust system for exhaust gas that connects a vehicle engine and a muffler to each other, a catalytic converter for purifying exhaust gas is generally provided. The engine may emit environmentally harmful materials such as CO, NOx, or unburned HC or VOC. In order to convert such harmful materials into environmentally acceptable materials, exhaust gas is caused to flow through a catalytic converter such that CO is converted into CO2, NOx is converted into N2 and O2, and VOC is burned to produce CO2 and H2O. In the catalytic converter, catalyst layers having a noble metal catalyst such as Rh, Pd, or Pt supported on a support are formed on cell wall surfaces of a substrate.
Examples of the support for supporting the noble metal catalyst include a CeO2—ZrO2 solid solution (also referred to as CZ material, cerium oxide (ceria)-zirconia composite oxide, and the like). This support is also called a co-catalyst and is an essential component of the three way catalyst which simultaneously removes harmful components in exhaust gas such as CO, NOx, and HC. Examples of an essential component of the co-catalyst include CeO2. The oxidation number of CeO2 changes into, for example, Ce3+ or Ce4+ depending on the oxygen partial pressure in exhaust gas to which CeO2 is exposed. In order to compensate for deficiency of charges, CeO2 has a function of adsorbing and desorbing oxygen and a function of storing oxygen (OSC: Oxygen Storage Capacity). In order to maintain a purification window of the three way catalyst, a variation in the atmosphere of exhaust gas is absorbed and alleviated such that the purification window can be maintained at substantially a theoretical air fuel ratio.
Incidentally, from the viewpoints of reducing material risk of rare metal and the like and obtaining cost competitiveness, how to decrease the amount of the noble metal catalyst used in the three way catalyst is an important factor. However, when the amount of the noble metal catalyst in the three way catalyst is significantly decreased, the catalytic activity is also significantly decreased. Consequently, the above-described OSC, low-temperature activity, NOx purification performance in a high-temperature environment, and the like are significantly decreased. The reason is as follows. Along with a significant decrease in the amount of the noble metal catalyst, the number of active sites is also significantly decreased, and the number of catalytic reaction sites is significantly decreased. As a result, a decrease in purification performance is significant.
Among the noble metal catalysts including Pt, Pd, and Rh which are particularly used in the three way catalyst, Rh has the highest NOx purification performance but has the highest market price per unit weight. In addition, it is known that Rh exhibits high OSC by being supported on a co-catalyst containing cerium oxide (ceria). However, it is also known that a trade-off relationship is established in that, as the amount of cerium oxide in the co-catalyst for supporting Rh increases, conversely, the NOx purification performance as a characteristic of Rh decreases. Therefore, when Rh is used as the noble metal catalyst in the three way catalyst, the design criteria during the preparation of the three way catalyst need to be set such that both the OSC and the NOx purification performance are simultaneously at an optimum.
In regard to the preparation of the optimum three way catalyst, in consideration of the fact that the performances of various catalytic noble metals and supports vary depending on the respective components thereof, a zone-coated catalyst, in which different components are disposed on an upstream side and a downstream side of a substrate so as to efficiently exhibit characteristics of the respective components, has been actively studied.
In regard to this zone-coated catalyst, PTL 1 discloses an exhaust gas purification catalyst including: a substrate for forming a gas passage through which exhaust gas flows; and catalyst layers that are formed on the substrate. More specifically, the catalyst layers applied herein include: a lower catalyst layer that is formed on a surface of the substrate; a front-upper catalyst layer with which a surface of the lower catalyst layer on an upstream side in a gas flow direction is coated; and a rear-upper catalyst layer with which a surface of the lower catalyst layer on a downstream side of the front-upper catalyst layer in the gas flow direction is coated. In addition, at least one of Pd and Pt is supported in the lower catalyst layer, Rh is supported in the rear-upper catalyst layer, Pd is supported in the front-upper catalyst layer, and a support for supporting Pd in the front-upper catalyst layer is a ZrO2 composite oxide containing Y2O3. According to this configuration, the purification characteristics of the catalytic noble metal can be sufficiently exhibited, and the low-temperature purification performance of the catalyst can be improved. In addition, by using the ZrO2 composite material to which Y2O3 is added, which has a low specific heat and superior heat resistance, as the support material of the front-upper catalyst layer, heat resistance can be secured while improving catalyst temperature rise performance, and catalyst warm-up performance including durability can be obtained.
On the other hand, PTL 2 discloses an exhaust gas purification catalyst including: a substrate; a lower catalyst layer that is formed on the substrate and contains at least one of Pd and Pt; and an upper catalyst layer that is formed on the lower catalyst layer and contains Rh, in which a region not including the upper catalyst layer is disposed on an exhaust gas upstream side of the exhaust gas purification catalyst, the lower catalyst layer includes a front-lower catalyst layer disposed on the exhaust gas upstream side and a rear-lower catalyst layer disposed on an exhaust gas downstream side, and the front-lower catalyst layer contains an oxygen storage material. According to this configuration, the grain growth of the respective catalytic metals supported in the respective catalyst layers, in particular, in the rear-lower catalyst layer and the upper catalyst layer on the exhaust gas downstream side can be significantly suppressed. Further, by providing the region not including upper catalyst layer on the exhaust gas upstream side, the diffusibility of HC to the inside of the front-lower catalyst layer can be improved, and the purification of HC in the front-lower catalyst layer is accelerated such that sufficient catalyst warm-up performance can be achieved.
Further, PTL 3 discloses an exhaust gas purification catalyst in which catalyst layers includes: a lower catalyst layer that is formed on a surface of a substrate; a front-upper catalyst layer with which a surface of the lower catalyst layer on an upstream side in a gas flow direction is coated; and a rear-upper catalyst layer with which a surface of the lower catalyst layer on a downstream side of the front-upper catalyst layer in the gas flow direction is coated. In the exhaust gas purification catalyst, at least one of Pd and Pt is supported in the lower catalyst layer, Pd is supported in the front-upper catalyst layer, Rh is supported in the rear-upper catalyst layer, and a concentration of Pd supported in the front-upper catalyst layer is 4.5 mass % to 12 mass %. According to this configuration, the purification characteristics of the catalytic noble metal can be sufficiently exhibited, and the low-temperature purification performance of the catalyst can be improved.
In this way, various techniques regarding the zone-coated catalyst are present. Under the above circumstances, the present inventors revised the configuration of the zone-coated catalyst and conceived a catalytic converter capable of obtaining superior NOx purification performance while reducing the amount of a noble metal catalyst.